Currently, medically intractable epilepsy afflicts about 1 million Americans. Unfortunately, most of the cases do not have a well definable focus, where the surgical removal of which could possibly eliminate the seizures. Implantation of subdural electrodes (SDEs) or depth electrodes (DEs) in patients with pharmaco-resistant epilepsy is a common strategy in epilepsy surgery and therapy that is employed when there is no discrete electro-physiologically abnormal lesion.
A central issue that occurs after implantation of the SDEs is the non-linear deformation of the brain surface by mass effect of the SDE array and due to the blood and fluid accumulation underneath the craniotomy flap used for the SDE implantation, as is seen on post-operative computed tomography (CT) scans, such as due to the blood and fluid accumulation underneath the craniotomy flap used for the SDE implantation. The deformation affects the localization of all electrodes, especially those under the bone flap, and causes them to appear to be depressed beneath the surface of the brain when visualized in a model created using co-registration of the post-implantation MRI scan with the a pre-implantation anatomical magnetic resonance imaging (MRI). As a result, some of the implanted electrodes may appear incorrectly in the model as being “buried” inside the brain rather than on the brain surface. Because the electrodes lying beyond the craniotomy flap are much less affected by this distortion problem, the unequal shift (displacement of the SDEs) renders inaccurate the localization based on mutual cost information algorithms that co-register the post-implant CT with the pre-implant MRI.
Attempts have been made to correct the displacement of the SDEs using semi-automated techniques to project “displaced” SDEs onto a high resolution MRI scan of the same patient. However, a significant error still exists with the current techniques for electrode localization. This error can be up to an 8 mm maximum error for a given electrode and a 4 mm mean error for all SDEs in a given individual. Given that the inter-electrode distance is usually 10 mm, an error of even 4 mm is substantial, and could possibly cause the electrode to be localized on an incorrect gyms, or even worse, an incorrect lobe.
The placement of DEs for localization of epilepsy was principally focused in years past only on limbic structures (hippocampus, amygdala and entorhinal cortex). Over the past few years the practice of stereo-electroencephalography (SEEG)—which involves the practice of placement of multiple (typically 8-16) DEs into myriad cortical and limbic targets in the brain has gained broader adoption. In its classical implementation this approach (designed in France in the 1960s) is performed using strictly orthogonal (lateral-to-medial) trajectories based on an arteriogram and using a stereotactic head frame. The limitation of this is that only certain trajectories can be accomplished and it is time consuming. The availability of three-dimensional frameless and frame based stereotactic navigation systems allow for the placement of these electrodes along azimuth-based trajectories. Such multi-directional trajectories allow for the placement of electrodes into any cortical structure. However, at the current time there are no good strategies to optimize the placement of these electrodes into “MRI-normal” or apparently non-lesional tissues that can generate seizures but are not abnormal on imaging. A “best-guess” approach is used by surgeons who place these electrodes, and who use approximations rather than patient specific anatomico-physiological boundaries.